1. Field of the Invention
The present invention relates to an exposure apparatus configured to expose a substrate to light through a liquid filled in a gap between the substrate and a final optical element of a projection optical system configured to project light through an original onto the substrate.
2. Description of the Related Art
Conventionally, a reduced projection exposure apparatus is used for manufacturing fine semiconductor devices such as a semiconductor memory or a logic circuit or liquid crystal display devices using a photolithography technique. The reduced projection exposure apparatus projects a circuit pattern formed on an original onto a substrate by a projection optical system and transfers the circuit pattern. The original is a reticle, a mask, or the like. The substrate is a semiconductor wafer, a glass substrate, or the like.
A minimum pattern size which can be transferred to the substrate by the reduced projection exposure apparatus (i.e., a resolution of the reduced projection exposure apparatus) is proportional to a wavelength of a light used in the exposure but is inversely proportional to a numerical aperture (NA) of the projection optical system. Thus, higher resolution can be achieved by reducing the wavelength or by increasing the NA. In recent years, according to a growing demand for higher-density circuit patterns, higher resolution capabilities have become important. Accordingly, increased NA of the projection optical system is expected as well as the implementation of reduced wavelength of the exposure light. Currently, development of the projection optical system with higher NA is making progress with accelerating speed and implementation of an optical system with the NA exceeding 0.9 is to start in the near future.
On the other hand, as a light source with shorter wavelength is demanded, argon fluoride (ArF) laser light source of a 193 nm-wavelength is used in place of krypton fluoride (KrF) laser light source of a 248 nm-wavelength. Currently, as a next generation light source, molecular fluorine (F2) laser light source of a 157 nm-wavelength or extreme ultraviolet (EUV) light source of a 13.5 nm-wavelength are being developed.
In the above circumstances, immersion exposure technology is receiving attention as a method for increasing resolution without changing the wavelength of the light source. The immersion exposure technology realizes increased NA by using liquid as a medium on a wafer side of the projection optical system. That is, a gap between a final face of the projection optical system and the wafer surface is filled with liquid. Since NA=n·sin θ, wherein n is the refractive index of the medium, the NA of the projection optical system can be up to n-times greater by filling the gap with a medium having a refractive index higher than that of air, i.e., n>1. Since the NA can be increased, the resolution can be improved as described above.
The immersion exposure method includes two methods: a local fill method in which a localized area of a wafer is immersed in liquid and a full field method by which a wafer surface is entirely immersed in liquid during exposure. The present invention relates to the local fill type exposure apparatus.
What is important in the immersion exposure of the local fill method is to completely fill the gap between the final face of the projection optical system or the final optical element, and an exposure area in the surface of the substrate (the wafer which is exposed) with liquid and to maintain this state during the exposure. The exposed substrate, such as a silicon wafer from which semiconductor chips such as an IC or a LSI are manufactured, is generally circular while the exposure area is generally rectangular. Thus, the above-described ideal immersion state is difficult to obtain when the exposure area is at the periphery of the wafer due to the curved configuration of the wafer. It is to be noted that in the following description, the exposure area may be referred to as an exposure target shot or an exposure target shot area.
For example, the volume of liquid that is necessary in filling the gap between the final optical element of the projection optical system and the wafer changes greatly when the exposure area is at the periphery of the wafer compared to when the exposure area is at the central area of the wafer. Thus, an ordinary immersion liquid supply mechanism is not practical in the exposure of the periphery of the wafer unless the amount of liquid supplied to the exposure shot area is changed according to its location. In other words, some complementary function is necessary in immersing an exposure shot area adjacent to a peripheral portion of the wafer in the conventional immersion exposure apparatus.